1. Field of the Invention
The field of the invention is that of telecommunication systems. To be more precise the present invention concerns a telecom network with on-demand dynamic resource assignment incorporating transmission via satellite. The satellite channels can be time-division multiplexed (TDMA) or frequency-division multiplexed (FDM) or, more generally, of the resource assignment type.
2. Description of the Prior Art
FIG. 1 shows a prior art satellite data transmission network. This network includes a satellite 11 and a communication resource management center 10 which communicates by radio with the satellite 11. Traffic stations 12, 13 comprising TDMA or FDM terminals also communicate with the satellite 11 and are connected to public or private telephone switching centers 14, 15, generically referred to herein as PSTN (Public Switched Telephone Networks). Each PSTN 14, 15 is connected to a plurality of users 16, 17.
Calls between users 16 and users 17 connected to different traffic stations are set up by the management center 10 which dynamically assigns transmission frequencies (in FDM mode) or time slots of a time frame (in TDMA mode) in response to call requests from users. This is known as demand assignment multiple access (DAMA) and this dynamic assignment of resources optimizes the use of satellite resources.
Satellite resources are thus assigned on-demand; when a user requests a call, and if this request can be met, a satellite channel is set up between the originating traffic station to which the requesting user is connected and a terminating traffic station to which the called party is connected. The management center 10 is also advised at the end of a call of the clearing down of the resources assigned to the call.
A communication system of this type is described in the article "Telephony applications on TDMA satellite systems" by B. BRINGER et al. in Commutation & Transmission, No 1/1992.
The center 10 handles not only management of satellite frequencies but also provision of modems at originating and terminating traffic stations in order to set up telephone calls.
In general terms, this system operates as follows:
In FDM mode the management center 10 assigns satellite frequencies when it detects line seizure by a user 16 or 17, this line seizure being indicated by an analog signal (specific frequency) or digital signal (line seizure signaling bit or word) transmitted by the user to the management center 10 via the PSTN 12 or 13. The PSTN 12 and 13 format the signals transmitted by the users for transmission to the management center 10 via a modem.
One example of a frame 20 of this kind is shown in FIG. 2. The frame 20 comprises 32 time slots IT1 through IT32 each of eight bits, the first time slot IT1 being reserved for special signaling, the time slot IT16 conveying line signaling output by the PSTN and the other time slots being reserved for transmission of user data (address digits, speech, etc) transmitted by the users in one transmission direction. These users are telephones, PABX or a public telephone network, for example. Each frame has a duration of 125 s and provides a communication bit rate of 2 Mbit/s.
The main drawback of a network of this type is that the network management center 10 must register and analyze call requests (address digits) in order to be able to assign the resources needed to set up calls. It also has to monitor calls, to bill users and to perform other ancillary functions (in fact, all the functions required of a transit center); its design is therefore highly complex.
The management center analyzes two types of signaling: line signaling and register signaling. Line signaling essentially comprises line seizure and line release signals and register signaling essentially comprises address digits and dialog between telephone switching centers.
The configuration of the management center must also be matched to its environment since the signaling exchanged differs from one country to another. Each manufacturer must therefore provide a number of management center designs equal to the number of countries in which they are marketed.
It might be considered feasible to apply code conversion to the signaling so that the latter is the same regardless of the signaling used in the transmission network (R2, No 5, No 7, Socotel, Colisle, Q.23, ISDN, ISPN, etc), which would enable the design of a generic management center; however, this solution would restrict the possibility for dialog between the PSTNs, and the development cost for all the various code conversion processes would be high. Also, converting the signaling code would slow down call set-up.
One object of the present invention is to remedy these drawbacks.
To be more precise, one object of the invention is to provide a transmission network in which the tasks of the management center are simplified and in particular in which the management center can be the same regardless of the signaling used in the communication network in which it is located. It is therefore possible to use the same management center regardless of the signaling used in the transmission network.
Another object of the invention is to provide a traffic station enabling the use of a generic management center of this kind.
A further object of the invention is to provide a transmission method suited to this system.